A known problem with trains, which can lead to disastrous results, is that on occasion whilst the train is travelling one or more wheels of the train do not rotate or rotate only partially. This usually occurs when the brake is applied. As the train travels, the non-rotating or sliding wheel slides along the rail, which in severe circumstances may lead to derailment of the train.
The brakes may be applied due to human error, for example the brakes may accidentally be left on by the operator. This happens most frequently with hand brakes, in which case some wheels may have a brake applied and lock up, while other wheels are rotating freely. Non-rotation of the wheels may also be caused by mechanical failure, for example failure of the pneumatic brakes, or collapse of the bearing preventing the wheel from rotating. In any case, failures would affect individual wheel sets or cars, and it is noted that in long trains this does not affect the ability of the train to travel. The reason for non-rotation or partial non-rotation may be varied, what is important is the detection of non-rotation or partial non-rotation.
If a wheel is not rotating, such that the wheel slides on the rail instead of rolling along it, the sliding wheel may develop a flat spot. The friction between the non-rotating wheel and the rail may cause the wheel to overheat. This in turn can cause the wheel to fracture, and a portion of the wheel may break off. In such cases, the axle of the fractured wheel may then become dislodged, damaged, or fractured, leading to derailment.
A derailed vehicle can damage infrastructure and rolling stock and also become a safety hazard for rail workers and the public. Non-rotating wheels may also cause other less catastrophic problems. For example, the rail head may be damaged, or the braking equipment may be damaged if non-rotation or partial rotation causes wheel flats which in turn may damage the brake blocks.
In order to guard against non-rotation or partial non-rotation of train wheels, it is currently necessary for a train examiner to stand next to the train as it travels past, and for the train examiner to conduct a visual check of the wheels. This can be referred to as roll-by inspection. It is vital that non-rotating wheels are detected during the time of inspection, as this is the only time that they can be checked. However, this method is not satisfactory, as a train examiner often fails to detect a wheel that does not rotate. This solution is also prone to human error, whether through inattentiveness, tiredness, poor visibility, boredom or any other reason.